1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Background Information
The tandem image forming apparatus has been conventionally produced as a full-color image forming apparatus. The tandem image forming apparatus generally includes a transport belt and units for forming black, magenta, cyan and yellow images (hereinafter simply referred to as “image forming units”). The image forming units are disposed along a direction that the transport belt transports a sheet of printing paper.
The tandem image forming apparatus is configured to form a color image by sequentially transferring toner images, formed on photosensitive bodies of the image forming units, onto a sheet of paper transported by the transport belt.
In the image forming apparatus of this type, the photosensitive bodies of the image forming units and the transport belt always make contact with each other.
In forming a monochrome image, only the black image forming unit forms a toner image whereas the magenta, cyan and yellow image forming units do not form any toner images. Nevertheless, all the photosensitive bodies of the image forming units and the transfer belt make contact with each other.
When a sheet of printing paper is transported while the transport belt and the photosensitive bodies make contact with each other, the photosensitive bodies may be damaged.
In response to the drawback, a mechanism has been proposed for separating color photosensitive bodies (i.e., photosensitive bodies of magenta, cyan and yellow image forming units) from a transport belt in printing a monochrome image and simultaneously making only a monochrome photosensitive body (i.e., a photosensitive body of a black image forming unit) press-contact to the transport belt.
As an example of the mechanism, the following image forming apparatus has been produced. In forming a full-color image, the image forming apparatus is configured to make a transport belt press-contact to photosensitive drums by transfer rollers used for transferring images of black, magenta, cyan and yellow images. In forming a monochrome image, on the other hand, the image forming apparatus is configured to release the press-contact condition between the transport belt and the photosensitive drums by displacing the transfer rollers downward. With the configuration, the image forming apparatus is capable of preventing the life cycle of the photosensitive drums from being reduced.
However, the conventional image forming apparatus has had a drawback that it takes long time to actually provide a user with printed material after the image forming apparatus receives a user's instruction of a print job. This is because the conventional image forming apparatus is configured to displace the transfer rollers when a print job (i.e., first job) is completed and subsequently a type (i.e., monochrome printing or color printing) of the next print job (i.e., second job) is determined. In other words, the conventional image forming apparatus is not capable of starting the second job until the displacement is completed.